The Respiratory System and Gas Exchange
The Journey Air Takes Into Your Body
Every time you inhale, air travels along a precisely ordered pathway before oxygen reaches your working muscles. Understanding this route is essential for explaining how the body meets the increased demand for oxygen during exercise. Air enters through the mouth or nose, where it is warmed, moistened and filtered. It then passes into the trachea — the windpipe — which is kept open by C-shaped rings of cartilage. From the trachea, air divides into the left and right bronchi (singular: bronchus), one entering each lung. Inside the lungs, the bronchi branch repeatedly into smaller and smaller tubes called bronchioles. The bronchioles finally open out into tiny air sacs called alveoli (singular: alveolus), and it is here that the vital process of gas exchange occurs.
The pathway of air follows five stages in sequence: mouth/nose → trachea → bronchi → bronchioles → alveoli. Commit this order to memory — it appears regularly in short-answer questions.
Gaseous Exchange at the Alveoli
Gaseous exchange is the movement of gases between the lungs and the bloodstream. It relies on diffusion — molecules naturally move from areas of high concentration to areas of low concentration. In the alveoli, the concentration of oxygen is high (freshly inhaled air) while the blood arriving from the body has a low oxygen concentration and a high concentration of carbon dioxide. As a result, oxygen diffuses across the alveolar wall into the blood, and carbon dioxide diffuses in the opposite direction, out of the blood and into the alveolus to be exhaled.
Six structural features of the alveoli are specifically designed to maximise the speed and efficiency of this exchange. First, the alveoli have a very large surface area in total — hundreds of millions of tiny sacs create an enormous surface for diffusion to occur across. Second, the alveolar walls are moist and only one cell thick, meaning the distance gases must travel is as short as possible. Third, the alveoli are surrounded by a rich network of capillaries, ensuring a continuous and large blood supply. Fourth, blood flowing through these capillaries is constantly replenished (deoxygenated blood arrives and oxygenated blood leaves), which maintains the concentration gradient that drives diffusion.
Once oxygen crosses into the blood, it binds with haemoglobin in the red blood cells to form oxyhaemoglobin. This oxygenated blood then travels via the pulmonary veins to the left side of the heart and is pumped out to the working muscles. Haemoglobin can also carry carbon dioxide on the return journey to the lungs.
A common misunderstanding is that oxygen delivery to muscles is purely about breathing harder. In fact, the rate of diffusion at the alveoli is limited by concentration gradients and surface area — you cannot increase the surface area of your alveoli by trying harder. What exercise does achieve is an increased breathing rate and deeper breaths (greater tidal volume), which refreshes the air in the alveoli more frequently, maintaining a strong concentration gradient. Additionally, more capillaries open up around active alveoli during exercise, increasing the effective surface area available for gas exchange. The structural adaptations of the alveoli — thin walls, large surface area, rich blood supply — are what make rapid exchange possible, not effort alone.
The Mechanics of Breathing
Breathing is an active mechanical process driven by muscles changing the volume and pressure inside the chest cavity. During inhalation at rest, the diaphragm contracts and moves downward, while the intercostal muscles contract to raise the rib cage upward and outward. These combined actions increase the volume of the chest cavity, which lowers the air pressure inside the lungs below atmospheric pressure. Air rushes in from outside to equalise the pressure — this is inhalation. During exhalation at rest, the diaphragm and intercostal muscles relax. The rib cage falls inward and downward under gravity, reducing the chest volume and increasing pressure in the lungs. Air is pushed out passively. Breathing at rest is therefore largely passive on the exhale.
During intense exercise, the body must move air much more forcefully and rapidly. On the inhale, accessory muscles come into play: the pectorals (chest muscles) and the sternocleidomastoid (a neck muscle) help to elevate the rib cage further and faster than the intercostals alone can manage. On the exhale, the abdominal muscles actively contract, pulling the rib cage down more quickly and forcefully expelling air. This ensures that stale air (rich in carbon dioxide) is cleared from the lungs faster, making room for fresh, oxygen-rich air.
Tidal Volume and Minute Ventilation
Tidal volume is the amount of air moved in or out with each breath. At rest, tidal volume is approximately 0.5 litres. During maximal exercise it can rise to 3 litres or more as the body draws on inspiratory and expiratory reserve volumes. On a spirometer trace, tidal volume appears as the regular rhythmic wave; the peaks and troughs become taller and more frequent as exercise intensity increases.
Minute ventilation is the total volume of air moved per minute and equals tidal volume multiplied by breathing rate. For example, a resting athlete breathing at 15 breaths per minute with a tidal volume of 0.5 litres has a minute ventilation of 7.5 litres per minute. During hard exercise, both tidal volume and breathing rate increase, so minute ventilation can rise to 100–150 litres per minute in trained performers. This enormous increase ensures the concentration gradient at the alveoli is maintained even as muscles consume oxygen at a high rate.
Six features make alveolar gas exchange efficient: large surface area, moist thin walls (one cell thick), short diffusion distance, large capillary network, large blood supply, and a maintained concentration gradient (high O2 low CO2 arriving from the body).
A spirometer trace records lung volumes over time. At rest, the trace shows a regular shallow wave — this is the tidal volume. Above the resting inhale is the inspiratory reserve volume (the extra air you could breathe in above a normal breath). Below the resting exhale is the expiratory reserve volume (the extra air you could push out). Below that is the residual volume — the air that always remains in the lungs and can never be exhaled. When exercise begins, the trace changes: the tidal volume wave gets taller (deeper breaths) and the waves appear more frequently (faster breathing rate). The residual volume remains unchanged throughout.